High-speed airfoil



May 16, 1950 c. E. PAPPAS ETAL HIGH-SPEED AIRFOIL Filed July 5, 1946 INVENTORS COSTAS E. PAPPAS AND HOSHEN R LU ATTORNEX Patented May 16, 1950 PATENT OFFICE HIGH-SPEED AIBFOIL Costas E. Pappas, Babylon, and Hoshen R. Lu, New York, N. Y., assignors to Republic Aviation Corporation, near Farmingdale, N. Y., a corporation of Delaware Application July 5, 1946, Serial No. 881,468

Claims. 1

This invention relates to ainfoils suitable for high speed airplanes and contemplates a construction wherein the critical Mach number is increased without an appreciable attendant increase in drag.

To that end the present airfoil proposes the reduction or relief of the crowding effect or congestion of the air particles following or flowing over the upper surface of the airfoil, which has, in the prior conventional high speed airfoils, resulted in shock waves and turbulence aft thereof at lower speeds than herein contemplated.

The instant invention also has in view a pressure distribution over the upper surface of the airfoil more nearly approaching the ideal distribution than has heretofore been considered possible.

With the above and other objects in view, as will be apparent, this invention consists in the construction, combination and arrangement of features, all as hereinafter more fully described, claimed and illustrated in the accompanying drawing, wherein is schematically disclosed an airfoil section or profile illustrating one adaptation in accordance with the teachings hereof.

It has been seriously advanced that the maximum speed possible with the conventional high speed airfoil of the prior art has been attained or at least is rapidly approaching attainment For example, the limit critical Mach number that can be expected for a 12% airfoil is in the neighborhood of 0.85 at a low value of the lift coefllcient, i. e. C1.=0.1. Therefore to realize the Mach numbers greater than 0.85 in a 12% airfoil the present airfoil is proposed.

In the conventional high speed airfoil heretofore employed the air particles flowing over or following the upper surface tend to congest or crowd producing a shock wave and turbulence aft thereof. By relievin this crowding effect or congestion through causing an appreciable portion of the air stream to flow from the leading edge through the airfoil section along the line of the chord to be discharged at and/ or adjacent the trailing edge, it is proposed to increase the critical Mach number without increasing the drag.

The ideal type or pattern of pressure distribution is rectangular or normal to the chord of the airfoil and presents a condition impossible of attainment or even approximation with the presently employed airfoil sections or profiles. By causing a flow of air from the leading edge through the body of the airfoil for discharge at two points, viz: the trailing edge and through the upper surface of the airfoil aft of the maximum thickness, the present airfoil more nearly attains a rectangular pressure distribution pattern than has heretofore been possible.

An airfoil construction in accordance with the teachings of the present invention is illustrated in the drawing and comprises an upper generally convex surface I! terminating forward in the leading edge I6 and aft in the trailing edge l8; and the lower or under generally convex surface 23 having a leading edge 20 and a trailing edge 22.

This airfoil at and along the chord a:x is divided by a fore and aft passage or duct it into an upper section ll outwardly defined by the upper airfoil surface l1 and inwardly by the upper side IQ of the passage or duct l3 and a lower section l2 outwardly defined by the lower or under airfoil surface 23 and inwardly by the lower side 2| of the passage or duct l3. The duct or conduit l3 extends completely through the airfoil chordwise and may be approximately co-extensive with the airfoil spanwise. Thus the airfoil, regardless of its specific details of construction or conformation, is separated by the passage or conduit l3, which may be said to generally coincide with the chord :v-:z: of the airfoil, into two sections l2 and H, the outer surfaces of which combine to create the envelope of the airfoil.

While the particular airfoil shape or profile forms no specific part of the present invention,

it has been determined that the best results will probably be obtained from a profile which embodies the well-known principle that the maximum thickness should be situated as far aft as possible for high speed operation. Thus with any profile regardless of its specific shape and form, by following the precepts of the present invention it will be possible to position or locate the maximum thickness of the airfoil further aft than heretofore possible without increasing the drag. This advantage is obtained by causing an appreciable part of the air stream at the leading edge of the airfoil to pass between the leading edges I8 and 20 respectively of the airfoil sections H and I2 and into the continuous passage l3--l3' between these sections for ultimate discharge, at least in part, between the trailing edges l8 and 22 of these air-foil sections.

Aft of the maximum thickness of the airfoil, the upper section It is traversed by a rear wardly curved and upwardly or outwardly extending auxiliary passage or duct 24 having a relatively large inlet throat 25 at the point where it Joins the main passage or duct I3I2'. This auxiliary conduit 24 extends upwardly and rearwardly through the upper section I4 of the airfoil to terminate and pierce, as at the slot 26, the upper surface I! of the airfoil and is so curved that it merges tangentially with the upper surface I'I. Aft of the extremity of the auxiliary passage 24 the outer surfaces of the afterbody I5 converge sharply and can, if desired, be somewhat concaved thereby producing relatively sharp trailing edges I8 and 22 for the upper and lower airfoil sections I2-I4.

It will be noted that the area of the opening or slot between the leading edges I6 and 20 respectively of the sections I4 and I2 greatly exceeds the area of the slot or opening between the trailing edges I8 and 22 respectively of the sections I4 and I2. To establish a confined and direct communication between these slots the passage or conduit I3-I3' generally tapers from the leading toward the trailing edge, the taper of the forward portion I3 thereof being relatively minor and that of the portion I3 aft of the throat or inlet 25 of the auxiliary conduit 24 being more pronounced.

It will be noticed also that the width of that part I3 of the passage I3 lying aft of the throat or inlet 25 of the auxiliary passage 24 is further reduced, so that a baille 34 is created at the junction between the reduced portion I3 of the passage I3 with the inner extremity of the auxiliary passage 24.

Should the aft portion I3 of the passage or duct I3 be of substantially the same cross-sectional area as that of the fore portion, the passage of the air from between the leading'edges I6-2l through the passage I3-I3' might result in internal draft or aspirator effect in the auxiliary passage 24 causing an inward flow of the upper air stream 28-29 rather than an outward air flow from the passage 24 to join the upper air stream. The reduction of the crosssectional area of the aft portion I3 of the chordwise conduit or passage creates the baflle 34 to project into the air stream flowing through the passage I3 and direct a portion thereof through the throat or inlet 25, the auxiliary passage 24 and the slot 26 in the upper surface I! of the airfoil. The undeflected portion of the air flowing between the leading edges I6 and 20 flows through the passage I3 and into the reduced aft portion or extension I3 to be discharged or exhausted as at 3|, between the trailing edges l8-22 of the airfoil.

That portion of the air flowing through the passage I3 which is deflected through the auxiliary passage or duct 24 and emitted through the slot 26 in the upper surface I1 joins or unites with the upper air stream 28-29 flowing over or following the contour of said upper airfoil surface H for ultimate delivery, as at 29, at the trailing edge I8-22 where it merges or unites with the air stream 32-33 flowing under or following the lower surface 23. The remainder of the air flowing through the chordwise passage I3-I3' attains an exit velocity at the trailing edge I3-22 in excess of the entrance velocity of the air stream 30 or any local velocity of the upper air stream 28-29 or of the lower air stream 32-33. Thus the air stream 3| discharged from between the trailing edges I8-22 of the airfoil sections I4I2 produces an induction effect that not only tends to reduce turbulence at the trailing edge and the drag resulting therefrom but acts directly upon the upper air stream 20 and the lower air stream 33. By providing two exits, viz: the slot 28 and the space between the trailing edges I3-22 for the escape or ejection of the air from the passage I3-I3', rectangular pressure distribution is more nearly approached than has been hitherto possible.

The exhaust or escape of the air from the aft portion I3 of the passage or conduit I3-I3' at {the trailing edge of the airfoil produces a boundary layer of control by virtue of the action of the air stream 3I from the passage I3 upon any air which has accumulated on the upper and lower airfoil surfaces "-23 adjoining the trailing edge Iii-22. As above pointed out the air flow 3| from between the trailing edges I8 and 22 of the sections I4-I2 has a greater exit velocity than the local velocity of the air stream 29 and 33 at the trailing edge I8-22 of the airfoil and this velocity acts directly upon the air flow over the upper and lower surfaces I'I-23 in that it tends to increase the velocity thereof and therefore prevents any congestion and crowding effect in the air streams 23 and 33 travelling over or following the upper and lower' airfoil surfaces I1 and 23 at or adjoining the trailing edges I8 and 22.

What is claimed is:

1. An airfoil having an intake opening at its leading edge and a smaller outlet opening at its trailing edge together with an auxiliary opening in its upper surface aft of its maximum thickness, a constantly open main duct situated on the chord of the airfoil and connecting the aforesaid intake and outlet openings, and a constantly open auxiliary duct extending outwardly and curving aftwardly from the main duct at or aft of the maximum thickness of the airfoil to connect with the auxiliary opening in the upper surface of the airfoil, the main duct tapering from the inlet opening to the outlet opening with an increase of its taper aft of the entrance to the auxiliary duct, and the auxiliary duct having a gradually decreasing cross-sectional area as it approaches the auxiliary opening in the upper surface of the airfoil.

2. An airfoil having spanwise slots at its leading and trailing edges, the slot at the trailing edge having a width less than the width of the slot at the leading edge, and a slot in its upper surface aft of its maximum thickness, a duct system, confined entirely within the limits of the airfoil, connecting said slots, comprising an aftwardly tapering main duct situated approximately on the chord of the airfoil connecting the slots at the leading and trailing edges thereof, and an auxiliary duct connecting the main duct aft of the maximum thickness of the airfoil with the slot located in the upper surface of the airfoil, said auxiliary duct having a throat of larger area than the area of the slot in the upper surface of the airfoil communicating with the main duct and tapering gradually from the main duct toward and communicating with the slot in the upper'surface of the airfoil.

3. An airfoil having spanwise slots at its leading and trailing edges with the slot at the trailing edge narrower than the slot at the leading edge and a slot in its upper surface aft of its maximum thickness, a duct system connecting said slots comprising a main duct, situated approximately on the chord of the airfoil, connecting the slots at the leading and trailing edges, an auxiliary duct having a relatively wide throat at its inner end communicating with the main duct aft of the maximum thicknes of the air- 5 foil and tapering upwardly and aftwardly from the main duct to communicate with the slot of the upper surface of the airfoil, the main duct having a gradual rearward taper forward of the maximum thickness of the airfoil and a more pronounced rearward taper aft of said maximum thickness.

4. An airfoil having spanwise slots at its leading and trailing edges with the slot at the trailing edge narrower than the slot at the leadin edge and a slot in its upper surface aft of the maximum thickness of the airfoil, a duct system connecting said slots comprising a main duct, connecting the slots at the leading and trailin edges, an auxiliary duct connecting the main duct with the slot in the upper surface of the airfoil, said auxiliary duct having a throat at its inner end communicating with the main duct aft of the maximum thickness of the airfoil of greater area than the area of the slot in the upper surface of the airfoil and tapering upwardly and curving aftwardly from the main duct to communicate with said slot of the upper surface of the airfoil, the main duct having a gradual rearward taper forward of the maximum thickness and a more pronounced rearward taper and less width aft of the maximum thickness, thereby creating a projecting baflie where the upper wall of the main duct joins the aft wall of the auxiliary duct.

5. An airfoil having open and unobstructed spanwise slots at its leading and trailing edges and in its upper surface aft of its maximum thickness, the slot at the leading edge being wider than the slot at the trailing edge, a constantly open duct system connecting said slots comprising a main duct tapering aftward from the slot at the leading edge to the slot at the trailing edge, the taper of said duct being greater and the width thereof less aft of the maximum thickness of the airfoil than forward thereof, and an auxiliary duct connecting the main duct at or aft of the maximum thickness of the airfoil with the slot in the upper surface of the airfoil. said auxiliary duct having an area where it joins the main duct greater than the area of the slot in the upper surface thereof, and tapering and curving upwardly and aftwardly from the main duct to said slot in the upper surface of the airfoil.

COSTAS E. PAPPAS.

HOSHEN R. LU.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,559,091 Hall Oct. 27, 1925 1,810,693 Alfaro June 16, 1931 2,078,854 Jones Apr. 27, 1937 2,136,403 Vance Nov. 15, 1938 2,267,927 Kightlinger Dec. 30, 1941 2,352,144 Woods June 20, 1944 FOREIGN PATENTS Number Country Date 738,478 France Oct. 17, 1932 OTHER REFERENCES Flight (Pub), Nov. 4, 1937, p. 450. (Copy in (Copy in Div. 

